In recent years, it has been desired that a fuel cell vehicle (FCV) be put into practical use as early as possible, since environmental problems and energy problems are increasingly attracting attention on a global scale. In this fuel cell vehicle, hydrogen and oxygen react with each other to generate electric energy and emit carbon dioxide (CO2) and water (H2O). Besides CO2 and H2O, hydrogen (H2) and methane (CH4) are emitted from a fuel cell stack and the like of the fuel cell vehicle using gasoline as a fuel. Of these, H2 is burned relatively easily even at low temperatures if a catalyst is used and can be emitted as H2O. Meanwhile, CH4 is one of the typical greenhouse effect gases in addition to CO2. One molecule of CH4 brings a global warming effect equivalent to that of approximately twenty molecules of CO2. Hence, it is necessary to control the emission of CH4 to the atmosphere as much as possible.
As a technology for removing CH4 from the exhaust gas, a catalytic combustion method has been known, in which CH4 is burned and removed by use of a catalyst under so-called lean conditions where oxygen is excessive.
Moreover, method for improving a catalyst is conceived in order to reduce CH4 in the exhaust gas. A catalyst for removing CH4 from the exhaust gas has been researched regarding the purification of the exhaust gas of a compressed natural gas (CNG) vehicle. Concretely, application of a three-way catalyst has been examined, and a technology for removing CH4 efficiently even under relatively low temperature conditions of approximately 400° C. has been disclosed (“Hanaki, Sekiba, Ishii, Okada and Ishizawa; Collected Papers of Society of Automotive Engineers of Japan, Inc., Vol. 27, No. 2, April, 1996 (9634549)”). This method is for controlling an air/fuel ratio of the exhaust gas at around a stoichiometric condition by use of a noble metal catalyst, and is conceived to be effective for purifying exhaust gas of an internal combustion engine, which contains a relatively little moisture.